Propagating plane wave electromagnetic radiation is composed of two orthogonally polarized components—designated as the transverse electric and transverse magnetic fields. In many applications, it may be necessary or desirable to separately control the amplitudes and relative phase of the transverse electric (TE) and the transverse magnetic (TM) polarizations. For example, device performance that varies based on polarization state may provide for multi-functioning opto-electronic devices.
Birefringence is a property of a material to divide electromagnetic radiation into its two components, and may be found in materials which have two different indices of refraction, referred to as n⊥ and n∥ (or np and ns), in different directions, often orthogonal. That is, light entering certain transparent materials, such as calcite, splits into two beams which travel at different speeds. Birefringence is also known as double refraction. Birefringence may serve to separate the two orthogonal polarizations, thereby allowing such devices to manipulate each polarization independently. For example, polarization may be used to provide add/drop capabilities, beamsplit incoming radiation or filter, by way of non-limiting example only. Birefringence may be caused by the anisotropic electrical properties of molecules, which form crystals. Alternatively, by forming patterns of three dimensional structures.
Anisotropic materials exhibit birefringence naturally in certain crystals such as hexagonal (such as calcite), tetragonal, and trigonal crystal classes generally characterized by having a unique axis of symmetry, called the optic axis, which imposes constraints upon the propagation of light beams within the crystal. Traditionally three materials are used for the production of polarizing components—calcite, crystal quartz and magnesium fluoride.
Generally, calcite is a widely preferred choice of material in birefringent applications, because of its high birefringence and wide spectral transmission, relative to other naturally occurring materials, though it is a fairly soft crystal and is easily scratched. Calcite, generally, has a birefringence approximately 0.172.
Quartz, another often useful birefringent material, is available as either natural crystals or as synthetic boules. Natural and synthetic quartz both exhibit low wavelength cutoffs—natural quartz transmits from 220 nm, while synthetic transmits from 190 nm—and both transmit out to the infrared. Quartz is very hard and strong thereby lending to the fabrication of very thin low order retardation plates. Unlike calcite or magnesium fluoride, quartz exhibits optical activity, and there is no unique direction (optic axis) down which ordinary and extraordinary beams propagate under one refractive index with the same velocity. Instead, the optic axis is the direction for which the two indices are closest: a beam propagates down it as two circularly polarized beams of opposite hand. This produces progressive optical rotation of an incident plane polarized beam, which effect may be put to use in rotators. Quartz has a birefringence on the order of 0.009.
Single crystal magnesium fluoride is another useful material for the production of polarizers, because of its wide spectral transmission. Single crystal magnesium fluoride has a birefringence of approximately 0.18.
Waveplates, also referred to as retarders, delay one of two orthogonally polarized components of light incident upon them. Waveplates are used in optical assemblies to alter the phase of light. Waveplates are generally asymmetric, and have a different refractive index in one axis than the other. Light polarized along the fast or optical axis encounters a smaller refractive index than light polarized perpendicular to this axis. The two orthogonal components of light, one polarized along the optical axis and one polarized perpendicular to that axis, traverse the wave plate and continuously acquire phase difference within the bulk of the material. For waveplates of ½ or ¼ wave delay, the two orthogonal components will emerge with a phase difference of π or π/2. In the case of a half waveplate, incident polarized light at an angle ⊖ to the optical axis is rotated by an angle 2⊖. A quarter waveplate causes linearly polarized light to become circularly polarized for incident polarization oriented at 45° with respect to the optical axis. Waveplates are characterized by bandwidth, defined as the range of wavelengths over which a device will operate, and order. Zero order waveplates generally have the largest bandwidth and as a result are preferred in applications that require wavelength tuning, or multiplexing, combining of light with substantially different wavelengths.